Duck beak check valves typically have a tubular inlet part that flattens towards its outlet—like the beak of a duck. In some duck beak valves the downstream parts are split at each side to form two opposing flaps.
Duck beak valves are used to control the influx of tidal waters into sewer and storm water outlets that discharge waste-water into open waters. They were also used as the check valve on World War II gas masks.
Current “duck beak” check valves rely on a superior downstream (“back”) pressure to close sealingly. If these valves are the inlet and outlet valves of a reciprocating pump that relies on creating a sub-atmospheric (vacuum) pressure in its inlet line to “lift” water into its inlet when pumping starts, back pressure on the valves can be insufficient to fully close the valve and produce an adequate seal, because the initially flat sealing surfaces tend, with time, to remain partly open.
In these circumstances the pump either fails to lift and begin pumping liquid, or it does so at a reduced rate.
In the prior art the “duck beak” in its closed configuration resembles a flattened tube at its outlet wherein a flat first inner surface sealingly contacts a flat second inner surface to provide a seal. In the absence of an adequate back pressure, it can be difficult to sustain the seal when the valve is closed. Stiffening of the tube walls near to the outlet, or closure of a part, or parts, of the outlet provide prior art remedies, but these means introduce disadvantages where the valve is to seal adequately while accommodating slurries containing large solids.
Prior art invention WO 2006/108219 provides means of reinforcing the duck beak to operate against a higher outlet pressure, but does not ameliorate the problem of decay causing the seal to become loose and ineffective over time.